This invention relates generally to electronic totalizers, and more particularly to an electronic totalizer that is interposable in the signal line of a two-wire transmitter.
A two-wire transmitter of the type useful in process control applications includes a field-mounted transducer which converts the metered process variable, such as liquid level or flow rate, into a current signal that is conveyed over the two-wire line to a remote receiving station. The receiver at the remote station may also perform algebraic computations upon the current signal, such as multiplying and dividing prior to its utilization by controllers, indicators or recorders.
One important advantage of a two-wire transmitter is that the same line serves not only to convey the current signal from the transmitter to the remote receiving station but also to deliver a direct-operating voltage from a power supply at the station to the transducer, thereby obviating the need for extra wires. The current output of a two-wire transmitter minimizes its susceptibility to voltage noise spikes and eliminates line drop problems.
A typical, commercially-available, two wire transmitter making use of linear integrated circuits is the model LH 0045/LH0045 C two-wire transmitter manufactured by National Semi-conductor Corporation and described in their instruction bulletin covering this transmitter.
In many industrial applications for a two-wire transmitter, the metered value in rate form must be totalized for billing or other purposes to determine the total flow that has passed through the meter. While a total flow reading may be obtained by means of a separate totalizing counter system associated with the readout located in the control room, it has not heretofore been possible to incorporate a totalizer in a two-wire transmitter and to power the totalizer directly from the line without adversely affecting the normal operation of the transmitter.